1. Field of the Invention
The present invention relates to a method of controlling an oxygen concentration sensor mounted in the exhaust system of an internal combustion engine for sensing the oxygen concentration in the exhaust gas of the engine.
2. Description of Background Information
Conventional systems for controlling the air/fuel use a feedback operation, in which the oxygen concentration in the exhaust gas of an engine is detected by an oxygen concentration sensor. The air/fuel ratio of the mixture being supplied to the engine is controlled to meet a target value by the feedback operation in response to an output signal of the oxygen concentration sensor. The purpose of this control scheme is to purify the exhaust gas and improve the fuel economy.
A typical oxygen concentration sensor used in such air/fuel ratio control systems is capable of producing an output signal whose level is proportional to the oxygen concentration in the exhaust gas of the engine. For example, in Japanese Patent Application Laid Open No. 59-192955, an oxygen concentration sensor is disclosed which includes a pair of flat oxygen ion conductive solid electrolyte members each of which is provided with a pair of electrodes. These members form an oxygen pump element and a sensor cell element respectively, and are arranged in a manner such that a gas retaining chamber acting as a gas diffusion restriction region, is formed between the oxygen pump element and the sensor cell element. Thus, the formed gas retaining chamber communicates with the flow of a gas under measurement through a communication hole, while the surface of the other electrode of the sensor cell element communicates with an atmospheric air chamber.
In this type of oxygen concentration sensor, a pump voltage according to a voltage difference between a voltage generated across the electrodes of the sensor cell element and a reference voltage is applied to the oxygen pump element. This pump voltage causes oxygen ions in the oxygen pump element to move toward the electrode facing the restriction region when the voltage generated across the electrodes of the sensor cell element is higher than the reference voltage. On the other hand, when the voltage difference between the voltage generated across the electrodes of the sensor cell element is equal to or lower than the reference voltage, the pump voltage is applied to the oxygen pump element causiing the oxygen ions, in the oxygen pump element, to move toward an outer electrode. By this operation, the value of a current flowing across the electrodes of the oxygen pump element, that is the pump current becomes proportional to the oxygen concentration in the gas supplied to the diffusion restriction region for air/fuel ratios both in the lean and rich regions.
When using oxygen concentration sensors of such a kind, it was discovered that the supply of an excessive pump current causes a blackening phenomenon. This is realized when the amount of oxygen is the diffusion restriction region is smaller than an amount which can be pumped out by the pump current, thereby causing oxygen to be removed fromthe solid electrolyte. When Zr (zirconium dioxide) is used as the solid electrolyte, oxygen O.sub.2 is removed from ZrO.sub.2 by supplying excessive current to the oxygen pump element, so that zirconium Zr is separated out.
Moreover, when using the oxygen concentration sensors of the oxygen-concentration proportional type which has been described above, it is necessary that the temperature of the oxygen concentration sensing element is sufficiently higher than (for example, higher than 650.degree. C.) the temperature of the exhaust gas under a steady operation of the engine to obtain an output signal characteristic which is proportional to the oxygen concentration. In other words, it is necessary to activate the solid electrolyte to cause a good conductivity of oxygen ions. Therefore, a heater element for heating the oxygen concentration sensing element is provided in an appropriate position with respect to the solid electrolyte. Thus, the feedback control of air/fuel ratio is executed by using the output signal of the oxygen concentration sensing element when the oxygen concentration sensing element is heated to a sufficiently high temperature. This type of air/fuel ratio control system is disclosed, for example, in U.S. Pat. No. 4,707,241.
Since generally the temperature of the oxygen concentration sensing element is low during the starting of the engine, it is desirable that the heating-up of the oxygen concentration sensing element is completed as quickly as possible. In addition, since the voltage generated across the electrodes of the sensor cell element is not raised sufficiently high under such a condition, there is a possibility that the pump current becomes excessive in the oxygen pump element such that the pump current is increased or decreased in accordance with a result obtained by comparing the voltage with the reference voltage.
Moreover, it is not possible to detect the temperature in the interior of the oxygen pump element and the sensor cell element. Therefore, conventionally it is estimated that an activated state, in which the oxygen pump element and the sensor cell element are heated by the heater element so that a desirable output characteristic of the proportional type is obtained, is reached when a predetermined time period has elapsed after the start of the supply of a heater current to the heater element. However, since the activated state is determined without respect to the actual temperature of the oxygen pump element and the sensor cell element, the activated state of the oxygen concentration sensor is not detected with enough accuracy.